Antibiotics are commonly used to target specific genes of both gram-positive and gram-negative bacteria and clear them before they can cause physiological damage. However, over the last two decades, the widespread use of certain antibiotics have led to antibiotic resistance in the target microbial genes, thereby severely limiting their clinical use (Peschel, 2002, Trends Microbiol. 10:179). The clinical world witnessed an alarming trend in which several gram-positive and gram-negative have become increasingly resistant to commonly used antibiotics, such as penicillin and vancomycin, which target the enzymes involved in the formation and integrity of bacterial outer membrane.
The discovery of linear anti-microbial proteins, such as the insect cecropins, and disulfide-bridged anti-microbial proteins, such as the defensins, initially raised hopes in anti-microbial therapy. Both cecropins and defensins have been evolutionarily conserved in invertebrates and vertebrates and constitute a major component of host innate immune defense (Boman, 2003, J. Int. Med. 254: 197-215; Raj & Dentino, FEMS Microbiol. Lett., 202, 9, 2002; Hancock The LANCET 1, 156, 201). Members of the cecropin and defensin families have been isolated from plants, insects, and mammals. They are normally stored in the cytoplasmic granules of plant, insect, and human cells and undergo release at the site of pathogen attack. Rather than targeting a specific enzyme, positively charged anti-microbial peptides interact with the negatively charged (and somewhat conserved) membrane components, i.e., membrane peptidoglycan (PGN) in gram-positive bacteria and lippopolysaccharide (LPS) in gram-negative bacteria.
Following the identification and initial characterization of the cecropins and defensins, it was anticipated that these peptides would not be subject to microbial resistance. However, it was soon discovered that both gram-positive and gram-negative bacteria can develop resistance against these anti-microbial proteins by modifying their membrane glycolipid components. These modifications probably weaken the initial interaction of these anti-microbial peptides with the membrane glycolipid and thereby significantly reduce their ability to form pores and lyse bacterial membrane.
Globally, one-fifth of potential crop yield is lost due plant diseases, primarily as a result of bacterial pathogens. Xylella fastidiosa (Xf) is a devastating bacterial pathogen that causes Pierce's Disease in grapevines (Davis et al., 1978, Science 199: 75-77), citrus variegated chlorosis (Chang et al., 1993, Curr. Microbiol. 27: 137-142), alfalfa dwarf disease (Goheen et al., 1973, Phytopathology 63: 341-345), and leaf scorch disease or dwarf syndromes in numerous other agriculturally significant plants, including almonds, coffee, and peach (Hopkins, 1989, Annu. Rev. Phytopathol. 27: 271-290; Wells et al., 1983, Phytopathology 73: 859-862; De Lima, et al., 1996, Fitopatologia Brasileira 21(3)). Although many agriculturally important plants are susceptible to diseases caused by Xf, in the majority of plants Xf behaves as a harmless endophyte (Purcell and Saunders, 1999, Plant Dis. 83: 825-830). Strains of Xf are genetically diverse and pathogenically specialized (Hendson, et al., 2001, Appl. Environ. Microbiol 67: 895-903). For example, certain strains cause disease in specific plants, while not in others. Additionally, some strains will colonize a host plant without causing the disease that a different Xf strain causes in the same plant.
Xf is acquired and transmitted to plants by leafhoppers of the Cicadellidae family and spittlebugs of the Cercropidae family (Purcell and Hopkins, 1996, Annu. Rev. Phytopathol. 34: 131-151). Once acquired by these insect vectors, Xf colonies form a biofilm of poorly attached Xf cells inside the insect foregut (Briansky et al., 1983, Phytopathology 73: 530-535; Purcell et al., 1979, Science 206: 839-841). Thereafter, the insect vector remains a host for Xf propagation and a source of transmission to plants (Hill and Purcell, 1997, Phytopathology 87: 1197-1201). In susceptible plants, Xf multiplies and spreads from the inoculation site into the xylem network, where it forms colonies that eventually occlude xylem vessels, blocking water transport.
Pierce's disease is an Xf-caused lethal disease of grapevines in North America through Central America, and has been reported in parts of northwestern South America. It is present in some California vineyards annually, and causes the most severe crop losses in Napa Valley and parts of the Central Valley. Pierce's Disease is efficiently transmitted by the glassy-winged sharpshooter insect vector. In California, the glassy-winged sharpshooter is expected to spread north into the citrus belt of the Central Valley and probably will become a permanent part of various habitats throughout northern California. It feeds and reproduces on a wide variety of trees, woody ornamentals and annuals in its region of origin, the southeastern United States. Crepe myrtle and sumac are especially preferred. It reproduces on Eucalyptus and coast live oaks in southern California.
Over the years, a great deal of effort has been focused on using insecticides to localize and eliminate the spread of this disease. However, there remains no effective treatment for Pierce's Disease. Other crops found in these regions of the State of California have also been effected, including the almond and oleander crops. The California Farm Bureau reports that there were 13 California counties infested with the glassy-winged sharpshooter in the year 2000, and that the threat to the State of California is $14 billion in crops, jobs, residential plants and trees, native plants, trees and habitats.